Let me take the "devil's advocate" stance I love so much (:roll:) and say why I don't think there's a problem with the monk.
Seriously though, I don't have a problem with them. But I've never classified them as "front line fighters." While the fighter, knight, or paladin are standing toe-to-toe with the big, bad beastie the monk, ranger, and barbarian are taking out ancillary targets or providing support to the main (heavy armor wearing) warriors. This is by no means a way to slight these latter classes but I see these types taking out the opposing wizard while the heavy-armor wearers slog through the masses of baddies. I see them more as specialized combatants who lend their expert touch when needed, but otherwise support the neavy hitters when necessary.
Now, I'm not saying my view is the only possible scenario. I'm just giving my two cents why I don't have an issue with them. I once ran a C&C game with a cleric, a monk, a ranger, and a wizard. So, it was interesting seeing the ranger and monk handle the "first-tier" fighter spot. It definitely can be done and I don't want to be misunderstood, by thinking I'm saying it can't.
But looking over at the class, I see them getting a lot of bonuses. So many, that I feel that if they start becoming equal to fighters in their ability to hit, I feel it takes away from the fighter the advantage of being a fighter. They don't have a ton going for them as it is. Make other classes approach their combat versility, then they become a less attractive (and viable?) choice.
I look at a 9th level monk. I like 9th level because, to me, it's the end of the low leve and the beginning of the high. So, it's a good spot for comparisons, in my opinion. So, looking at the 9th level monk, it's a very interesting scene.
d12 hit die? Check.
Saving throw bonuses to disease, poison, paralysis, polymorph, petrification, death magic, confusion, charm, fear, and mind-affecting spells? Check.
The ability to stun foes? Check.
Deflect missiles? Check.
Improved healing over the other classes? Check.
Fall 30' without taking damage? Check.
Hit magicially fortified creatures? Check.
At 9th level, they're hitting AC 18 (19 or 20, with a decent strength) quite consistantly. Give them a Amulet of Mighty Fists, as others suggested, to counteract the fighter's
+2 longsword, and you shouldn't need to toy with anything.
I feel that if you were to improve upon their ability to strike at foes - an an inherent class ability - to near the level of a fighter, then you start infringing on the fighters' main area of expertise.